Preheat oven to 400 degrees.
One thing to note before you start mixing, your cast iron will need to go in the oven for at least 5 minutes before you are ready to bake. You want it to be good and hot before the batters go in, so keep that in mind when you are prepping your ingredients.
It is important to make sure everything is ready to go before you start mixing. It should come together really quickly. So, cut up your jalapenos and grate your cheese. Then mix together the flour, baking powder, salt, and sugar. In another bowl, mix your eggs, milk, oil, and butter.
Combine the wet and dry, remember to only mix to combine. You don't need to overdo this! Add in your cheddar and jalapenos. Mix to combine.
Remove your hot cast iron from the oven and grease generously with butter. Pour in the batter and place immediately in the oven.
Bake for 15-25 minutes. This will depend on the actual heat of your oven. I check for a few things when baking- one is color. You are looking for that beautiful golden brown color, the edges will be slightly darker. I also check by touch. If you press gently in the center of the cornbread you will be able to tell whether or not it is too underbaked. The last way to check is with a toothpick. Just remember, you want it to be SLIGHTLY underbaked, so you don't want the toothpick to come out completely clean. Just mostly.
When it is cooked, remove it from the oven and allow it to cool for about 5-10 minutes before serving. You want it to finish baking to perfection.